Abstract
Climate change is expected to cause a poleward shift of many temperate species, however, a mechanistic understanding of how temperature and species' life histories interact to produce observed adult range is often lacking. We evaluated the hypothesis that juvenile thermal tolerance determines northern range in gray snapper (Lutjanus griseus), a species commonly caught as juveniles along the US Atlantic coast well north of their adult distribution, using a combined laboratory, field and modeling approach. The cumulative degree days below 17°C (CDD<17) survived by individuals maintained in the laboratory under ambient seasonal cooling conditions were used to quantify the chronic effects of prolonged but non-lethal low temperatures. Under ambient conditions, juveniles stopped feeding below 11.3°C, and the mean temperature at death was 10.2°C (+/−0.74S.D.). The CDD<17 endured by individuals ranged from 61.9 to 138.8 (83.54+/−22.44S.D.), and was positively related to fish size. The relation between CDD<17 and fish size was extrapolated to the maximum juvenile size in fall to provide an upper limit for survival in the wild (CDD<17=210days). Acute low temperature tolerance was evaluated in a second experiment that exposed juveniles to a constant rate of temperature decline (3°Cd−1), where feeding ceased at 10.3°C and death occurred at 7.0°C (+/−0.45S.D.). These thermal thresholds were combined with winter estuarine temperature data at 12 sites to evaluate potential overwinter survival along the US Atlantic coast. Of a total of 134 site-year combinations categorized based on the acute and chronic thresholds, the CDD<17 threshold (>210days) was more limiting than the acute threshold (minimum daily temperature<7.0°C) to survival of juveniles for a given site-year. To evaluate the relationship between juvenile thermal tolerance criteria and adult distributions, we quantified adult distribution using field observations in the western Atlantic Ocean compiled from a database of recreational divers. There was a strong correspondence between observations of adult gray snapper from the database of recreational divers vs. latitude with that of the predicted survival of juveniles vs. latitude from our analysis. The agreement between the laboratory-derived thermal tolerance metrics, the spatial distribution of winter temperature, and the distribution of adult gray snapper support the hypothesis that the adult range of gray snapper is largely limited by the overwinter survival of juveniles. Understanding the interaction between physiology and range is important for forecasting the impacts of climate change on other species of fish where juvenile tolerances are critical in determining range, particularly in seasonal systems.
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► Early life stages of gray snapper extend well poleward of the adult distribution. ► We posit temperature related overwinter mortality of juveniles determines adult range. ► Acute and chronic thermal tolerances were 7°C and 210°days below 17°C. ► The latitudinal extent of these thermal limits matches adult distribution.